Merge branch '4.4'

* 4.4:
  [Console] Constant STDOUT might be undefined.
  Add missing conflict with symfony/serializer <4.4
  Allow returning null from NormalizerInterface::normalize
  bumped Symfony version to 4.4.0
  updated VERSION for 4.4.0-BETA1
  updated CHANGELOG for 4.4.0-BETA1
  [Security\Core] throw AccessDeniedException when switch user fails
  [Mime] fix guessing mime-types of files with leading dash
  [HttpFoundation] fix guessing mime-types of files with leading dash
  [VarExporter] fix exporting some strings
  [Cache] forbid serializing AbstractAdapter and TagAwareAdapter instances
  Use constant time comparison in UriSigner

Terminal.php

@@ -98,7 +98,7 @@ private static function initDimensions()
      */
     private static function hasVt100Support(): bool
     {
-        return \function_exists('sapi_windows_vt100_support') && sapi_windows_vt100_support(STDOUT);
+        return \function_exists('sapi_windows_vt100_support') && sapi_windows_vt100_support(fopen('php://stdout', 'w'));
     }
 
     /**